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Most relatively large (> 250 ha) and shallow (< 2 m), Dutch lakes originate from large-scale peat mining. Originally, their water tables were regulated by natural processes such as evapotransporation, precipitation, drainage and seepage (up and downwards). These lakes used to be oligo to mesotrophic, and the phytoplankton consisted mainly of diatoms and green algae. However, 30 years ago filamentous cyanobacteria were also present. Owing to the intensification of agriculture in the surroundings of the lakes, their management developed more and more towards a man-made reservoir function. This management resulted in inflow of eutrophic water from the agricultural areas in winter. In summer the lakes were increasingly used as a water supply. This supply is compensated by water imported mainly from the River Rhine. The impacts of this reservoir function on the limnology of the Dutch fen lakes are discussed with the Tjeukemeer, the Loosdrechtse Plassen and the Reeuwijkse Plassen as examples. All these lakes have become typical eutrophic waters with high turbidity, dominance of filamentous cyanobacteria and bream and complete loss of submerged macrophytes. Finally, methods and results of manipulation procedures to restore these waters are discussed.

Urania basin in the deep Mediterranean Sea houses a lake that is >100 m deep, devoid of oxygen, 6 times more saline than seawater, and has very high levels of methane and particularly sulfide (up to 16 mM), making it among the most sulfidic water bodies on Earth. Along the depth profile there are 2 chemoclines, a steep one with the overlying oxic seawater, and another between anoxic brines of different density, where gradients of salinity, electron donors and acceptors occur. To identify and differentiate the microbes and processes contributing to the turnover of organic matter and sulfide along the water column, these chemoclines were sampled at a high resolution. Bacterial cell numbers increased up to a hundredfold in the chemoclines as a consequence of elevated nutrient availability, with higher numbers in the upper interface where redox gradient was steeper. Bacterial and archaeal communities, analyzed by DNA fingerprinting, 16S rRNA gene libraries, activity measurements, and cultivation, were highly stratified and metabolically more active along the chemoclines compared with seawater or the uniformly hypersaline brines. Detailed analysis of 16S rRNA gene sequences revealed that in both chemoclines δ- and ε-Proteobacteria, predominantly sulfate reducers and sulfur oxidizers, respectively, were the dominant bacteria. In the deepest layers of the basin MSBL1, putatively responsible for methanogenesis, dominated among archaea. The data suggest that the complex microbial community is adapted to the basin's extreme chemistry, and the elevated biomass is driven largely by sulfur cycling and methanogenesis.